Methods for preparing an organic silver salt dispersion, which is used as a silver source in thermally processable photosensitive materials, are well known in the art, including a method of preparing organic silver salts in the concurrent presence of water and an organic solvent, as described in JP-A 49-93310, 49-94619 and 53-68702 (herein, the term, JP-A means published unexamined Japanese Patent Application); a method of preparing organic silver salts in an organic solvent, as described in JP-A 57-186745 and 47-9432, and U.S. Pat. No. 3,700,458; and a method of preparing organic silver salts in an aqueous solution, as described in JP-A 53-31611, 54-4117, 54-46709 and U.S. Pat. No. 5,434,043.
There are also known methods for incorporating silver halide into a thermally processable material to provide photosensitivity to the thermally processable material. There have been employed silver halides prepared based on a process of preparing conventional aqueous silver halide gelatin emulsions, which are advantageous in terms of easy introduction of techniques, such as control of the crystal size and crystal habit, impurity doping and chemical ripening. In cases when preparing organic silver salts concurrently in the presence of an organic solvent or in an organic solvent solution, however, it is difficult to allow the silver halide, which has been made water-miscible through gelatin, to be homogeneously dispersed in the organic silver salt dispersion, making it hard to achieve desired photographic characteristics, such as maximum density and sensitivity. Accordingly, in general, silver halide is added at the stage of preparing an organic silver salt in an aqueous solution. Thus, it is advantageous to prepare an organic silver salt dispersion in an aqueous solution.
In the process of preparing an organic silver salt dispersion in an aqueous solution, a fatty acid is dissolved in water at a temperature of higher than its melting point to form a sodium salt, then, after the temperature is lowered, a silver halide emulsion is added and silver nitrate is further added thereto to form an organic silver salt dispersion. Lowering the temperature before adding silver nitrate results in a rapid increase in solution viscosity and addition of silver nitrate also increases the viscosity. In cases when the solution was not uniformly stirred at a high speed, the organic silver salt was non-uniformly formed so that monodispersed fine organic silver salts were not obtained, a thermally processable material prepared by use thereof resulting in problems in performance, such as high fogging levels. It has therefore been desired to solve the problems described above.